In present-day microprocessor based systems, it is common practice to utilize custom-built printed circuit cards or boards to control and/or monitor external events and conditions. These customized cards, along with the microprocessor card, are arranged to be plugged into select positions or discrete slots on a motherboard of a bus oriented system. Each of the cards carries the necessary electronics for interfacing with the bus line and for controlling and monitoring the operations of external devices. The motherboard is designed to enable the microprocessor to pass the address, data and control signals to associated peripheral printed circuit cards. The input signals are appropriately distributed at select locations and along the motherboard by employing a bus line cable. In the past, all of the input signals on any given bus line were capable of being conveyed by a plug-in connector of a number of slot insert cards unless special precautions were taken to prevent erroneous insertion of a bus plug with a printed circuit card receptacle. In practice, each printed circuit card has the ability to decode and process the information carried by the bus line. The information ingested is dependent upon the particular functional configuration of the card and is controlled by the bus interface electronics carried by each card.
Since the bus is a comprehensive element in the system, any number of cards having the appropriate interface electronic can be interconnected to the bus. Each card is coded or assigned a distinct address and is capable of decoding its address when powered by a bus line. The address identifies each particular card according to the function that it is to perform in operation. Presently, the methods of address decoding do not provide any means for dynamically selecting or altering the particular address assigned to a card. In most cases, the address code is either permanently fixed in the bus interface electronics or is electrically established by jumper wires connected to the card. Thus, if it is desired to modify the address, it is necessary either to replace the electronics hardware or to remove the card from the system so that the jumpers can be changed to set up different address codes. It will be appreciated that these previous methods of modifying the address codes are both time consuming and relatively costly.